ACID DYE

Definition

Dye which is mostly sulphuric or carboxylic acid salt and essentially applied from acidic or neutral dye bath known as Acid dye. The dye anion is the active colored component in these dyes, they are synthesized as sodium salts as free dye acids are more difficult to isolate.

Properties

·         These dyes are anionic in nature.

·         These dyes are soluble in water.

·         These dyes are suitable for wool, silk, polyamide and modified acrylics.

·         These are applied from a strongly acidic to neutral pH bath.

·         Generally, the molecular weights of acid dye ranges from 200-900.

·         Generally, dyes contain 1 to 4 sulphonic groups present in the structures.

·         These dyes have no affinity for cotton cellulose’s, hence not suitable for cellulosics.

·         These dyes combine with the fiber by hydrogen bonds, vander waals forces or through ionic linkages

Classification:

Structure

These dyes are normally very complex in structure but have large aromatic molecules, having a sulphonyl or amino group which makes them soluble in water. Most of the acid dyes belong to following three main structural molecules,      

1.       Anthraquinon type

2.       Azo dye type

3.       Triphenylmethane type

Dyeing characteristics

	Types of Dye
Property	Self – Levelling	Milling	Super-milling
Color brightness	Good	Lower than leveling	Fair (deep shade)
Levelling	Very good	Poor to fair	Very poor to fair
Affinity	Less	High	Very high
Migration	Excellent	Poor	Very poor
Wet fastness	Low	Good	Very good
Light fastness	Very good	Good	Good
Dyebath additives (pH adjusting agent)	Sulphuric or formic acid	Acetic acid	Ammonium acetate
Dyeing pH	2 – 4	4 – 6	6 – 7
Molecular weight (g/mole)	Low (200-400)	High (500-900)	High (500-900)
Molecular Size	Very small	Relatively bigger	Biggest
Water solubility (g/l)	High (40-80)	Moderate (13-30)	Low (3-20)
Solution behaviour	Ionizes	Aggregates	Aggregates
Protein affinity	Low	High	Very high
Cellulose staining	None	Stains	Stains

Mechanism

(1)   Dye: On the addition of dye in aqueous solution, it produced colored anion as follows

(2)   Fiber: When protein and polyamide fibers immersed in water, H-atom attached to the carboxylic group at one end of fiber and transferred to –NH₂ group at the other end of the macromolecule so that the two ends of fiber chain acquire opposite electrical charges, called zwitter ions:

 

(3)   Dye and Fiber: These cationic sites are thus available for the acid dye anions to combine with through hydrogen bonding, vander waals forces or ionic bonding in acidic condition. These linkages are strong enough to break, and thus dyeing produced is fast.  

Reaction between an acid dye and fiber can be represented by following equation

 

Problems with acid dye

           

·    Unequal access of the fibers to the dye solution, resulting from densely packed fibers or yarns and from poor agitation of the dyebath.

·       Variation of the temperature throughout the dyebath and the goods.

·       Uneven pH in the bath and the material.

References:

1.      Chemical Technology in the Coloration of Textiles, S.R.Karmakar(2007), 92-96

2.      Fundamental & Practices in Coloration of Textiles, J.N.Chakraborty(2010), 166-169

3.      Textile Preparation & Dyeing, AKR Choudhury(2011), 470-475

4.      Handbook of Textile & Industrial Dyeing, M.Clark(2011), 164-167

PARA – ARAMID FIBER (KEVLAR)

             

In this group fall the para – aramid fibers based upon poly (paraphenylene terephthalamide) (PPT), notably Kevlar (dupont) and Twaron (Enka) and the polybenzobisthiazole (PBZT) and polybenzobisoxazole (PBZO) fiber which possesses para – linked heterocyclic structures free from amide groups, and therefore fall outside the aramid category. These are all solution spun using an air – gap wet spinning technique from nematic liquid crystalline solutions.

Production

Kevlar is synthesized from the monomers 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride. The result is a polymeric aromatic amide (aramid) with alternating benzene rings and amide groups. When they are produced, these polymer strands are aligned randomly. To make Kevlar, they are dissolved and spun, causing the polymer chains to orient in the direction of the fiber.

Kevlar has a high price at least partly because of the difficulties caused by the use of concentrated sulfuric acid in its manufacture. These harsh conditions are needed to keep the highly insoluble polymer in solution during synthesis and spinning.The chemical synthesis of kevlar from 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride.

Para – Aramid Fibers Properties

ð  Density                        :           1.45 g/cc

ð  Melting Temperature   :           500°C

ð  Solvent                        :           H₂SO₄

ð  Tenacity (g/d)              :           Dry :- 8 – 22  ;  Wet :- 7 – 21

ð  Elongation (%)            :           8 – 20

ð  Modulus (g/d)             :           120 – 130

ð  Recovery (%)              :           100

ð  Moisture Uptake (%)  :           3.0 %

ð  At elevated temperatures, the aromatic polyamides have greater mechanical properties.

ð  Kevlar fibe is one of the strongest of man – made fibers.

ð  Dimensional stability of Kevlar is excellent. There is negligible change in length in hot water or in air at temperature as high as 160°C.

ð  Kevlar is susceptible to photo – degradation.

ð  Kevlar is unaffected by most organic solvents but it is attacked by strong acids and bases at high concentration or at high temperatures.

Applications of Para – Aramid Fibers

ð  They are being used in some designs of helicopters and light weight fighter planes.

ð  Its lower creep and excellent creep rupture characteristics are important in the use of aramids as filament wound, pressure vessel reinforcement.

ð  Protective gloves, aprons, vests, helmets, jackets, boots etc. are developed utilizing the cut and penetration resistance of these high performance fibers.

ð  Aramid fibers are providing improved damage tolerance in off – shore power boats, base boats, ocean racing yachts, small sail boats, canoes and kayaks.

ð  In sporting goods aramid fibers are applied such as golf shafts, archery bows, paddles, tennis rackets, fishing rods, track and field equipment.

ð  Aramid fibers are also used to manufacture ropes and cables with high strength and low stretch, such as wire ropes and electromechanical cables.

High strength and modulus of aramid fibers has led to their use in a number of rubber (tires, v – belts, hose etc.), plastic (epoxy, polyester,PVC etc.) and elastomer (neoprene, urethane etc.) reinforcement applications.

Printing of Polyester by Discharge Style

Principle     

·         Polyester fabric is dyed with dischargeable disperse dye and then printed with paste containing reducing agent for white discharge and non-dischargeable dyes for colour discharge.

·         Discharge printing of polyester is very difficult, because the dye is dissolves in polyester fabric.

·         If the fabric is dyed with HTHP method, then it is very difficult to discharge the ground.

·         But if the ground shade is obtained by carrier dyeing, then it is easy to discharge ground shade.

Dyeing

·         The cloth is first padded with a dispersion of the dischargeable dye and dried at low temperature so that colour does not get fixed at this stage, also the bath contains 2 g/l sodium alginate made acidic with tartaric acid (pH 5 – 6), then dry the fabric at 60°C and printed with a paste.

Procedure

For White Discharge

·         The printing paste is prepared as follows:-

     200 parts    Safolin

                               70 parts    Water

                             650 parts    Loust bean gum (10 %) or Meyprogum (8 %)

                               30 parts    Citric acid or (1 – 2 parts Tartaric acid)

                               50 parts    Thiodiethylene Glycol

1000parts

For Colour Discharge

·         The printing paste is prepared as follows:-

       50 parts    Disperse Dye (non-dischargeable)

     200 parts    Stannous Chloride

                               70 parts    Water

                             600 parts    Loust bean gum (10 %) or Meyprogum (8 %)

                               30 parts    Citric acid or (1 – 2 parts Tartaric acid)

50    parts   Sodium Thoicynate

1000 parts

After printing, fabric is dried and steamed for 30 minutes at 128ºC (30 psi pressure). This system produces full colour yield and bright prints.

After fixation, fabric is rinsed with cold water and hot water, reduction cleared (at 40 – 50ºC for 15minutes) in following bath:-

20 parts           Sodium hydrosulphite

30 parts           Sodium hydroxide

                                            1000 parts

·         Finally, the cloth is rinsed, soaped at the boil, washed and dried.